A user equipment (UE) in a mobile communications network can be in an idle state or a connected state. A UE that is in a connected state maintains its radio connection with the network, and the UE is able to exchange user plane data with the network over the radio connection.
A UE in the idle state does not maintain a radio connection with the network, and thus is unable to exchange user plane data with the network while the UE is in the idle state. The network can release radio resources associated with the UE when the network instructs the UE to transition from a connected state to an idle state.
A random access procedure can be performed by a UE to associate with the network and to acquire resources in the network in various scenarios. In a first scenario, the UE can initiate a random access procedure when the UE is in its idle state and the UE requires a connection to the network because either a paging message has been sent from the network or due to the arrival of uplink data that is to be sent to the network. In a second scenario, a random access procedure can be performed as part of handover from a first cell to a second cell. There can be other scenarios in which the UE can initiate a random access procedure.